Iron is the most abundant trace mineral in the body with the average person containing roughly 5 mg. Iron is rapidly metabolised and the high turnover of iron results in substantial daily losses explaining the high incidence of iron deficient anaemia Worldwide. Intakes of around 15 mg per day are required to prevent anaemia, but this amount is dependent on the form of iron taken. Haem iron is the form of iron found in animals, and includes that present in myoglobin and haemoglobin. Haem iron is generally well absorbed and the preferable form of dietary iron in human nutrition. In contrast, non-haem iron is the form found in plants, and this form of iron is not well absorbed in humans. A number of inhibitors of non-haem iron exists, and these can reduce absorption further. Soy proteins for example have been shown to decrease the absorption of iron significantly. The presence of inhibitors in legumes such as soybeans may therefore explain why beans are a poor source of iron despite having a reasonably high iron content.
However, it is possible to increase the absorption of iron from legumes and other plant sources. Experiments have been performed and a number of enhancers of iron absorption are known. For example, in one study1, researchers fed healthy subjects a meal containing either isolated soy protein as a liquidised drink with other ingredients or cooked or uncooked whole soybeans. The absorption of iron from the uncooked soybeans was 0.64 % but this rose to between 1.28 and 1.60 % with cooking. The absorption of iron from the soy protein isolate liquid meal was also very low at 0.56 %, but this rose to 3.2 % with the addition of 100 mg of ascorbic acid (vitamin C). In contrast the iron absorption from a meal containing egg albumin protein was 5.05 % and this rose to 10.19 % with the addition of 100 mg of ascorbic acid. When beef was added to the soy protein isolate meal, absorption increased from 0.36 to 1.44 %. However, when beef was added to the egg albumin meal, absorption of iron increased from 5.94 to 7.47 %.
Ascorbic acid and meat therefore strong enhancer non-haem iron absorption, and cooking appears effective at increasing the absorption to some extent. Nutritionist have long known that ascorbic acid will enhance iron absorption, and vitamin C is often found alongside non-haem iron in supplements. The increase in bioavailability of non-haem iron with addition of meat has been demonstrated elsewhere and is interesting. Low molecular weight protein digestion fractions of beef protein may form complexes with the non-haem iron. This may then increase the absorption of the iron across the brush border and into the enterocytes. Generally the poor absorption of non-haem iron is thought to relate to the fact that the iron is in the oxidised Fe3+ form, which is not readily absorbed. Vitamin C can reduce iron to its Fe2+ form, a form that is much more bioavailable. Cysteine residues on proteins may also reduce the Fe3+ to Fe2+ iron and thus contribute to the improved iron bioavailability of meat.
Dr Robert Barrington’s Nutritional Comments: Plant sources of iron are poorly absorbed and may not be suitable for restoring iron status in cases of iron deficient anaemia without great care. Legumes appear to contain inhibitors of iron absorption which further reduces its bioavailability. Cooking legumes may increase the bioavailability of the iron somewhat through destruction of the phytochemicals that inhibit its absorption. However, addition of both ascorbic acid and beef protein to legumes increases iron absorption substantially. Haem form of iron should always be chosen over non-haem forms where possible. If supplements are required then those with the iron chelated to organic compounds, such as iron glycinate or iron bis-glycinate, should be chosen over inorganic forms such as ferrous sulphate.
RdB
